Pneumatic tire for heavy load

ABSTRACT

In a heavy duty pneumatic tire comprising a carcass layer, an innerliner layer and an inner face protection layer arranged therebetween, the inner face protection layer is comprised of a rubber layer A and a rubber layer B, and a rubber composition constituting each layer is compounded with a rubber component, sulfur and a cobalt compound of an organic acid, and an amount of sulfur compounded satisfies the following equations (I) and (II): 
 
S A &lt;S B ≦S C   (I) 
 
2≦S A ≦4  (II) 
 
(wherein S A , S B  and S C  are an amount of sulfur compounded in a rubber composition constituting the rubber layer A, rubber layer B and the carcass layer, respectively, based on 100 parts by mass of the rubber component), and such a heavy duty pneumatic tire suppresses the creeping of the carcass rubber and also prevents the breakage of the inner face protection layer in the use at high-temperature, humid areas.

TECHNICAL FIELD

This invention relates to a heavy duty pneumatic tire, and moreparticularly to a heavy duty pneumatic tire considerably improving adurability in use at warm and humid areas.

BACKGROUND ART

In the conventional heavy duty pneumatic tire, an innerliner layer madefrom a butyl based rubber composition having a low air permeability isarranged as an innermost layer for holding an air pressure of the tire,and also an inner face protection layer is arranged between theinnerliner layer and a carcass layer. The term “carcass layer” usedherein means a carcass rubber portion of a carcass ply comprised of plycords and a carcass rubber covering these ply cords.

The inner face protection layer is required to have a function oftrapping oxygen so as not to arrive oxygen passing through theinnerliner layer at the carcass ply to deteriorate an adhesion forcebetween the ply cord and the carcass rubber, and a function ofreinforcing the carcass rubber so as not to creep the carcass rubberoutward in a radial direction due to the running of the tire or theinternal tire pressure to stand out the ply cords toward the inner faceof the tire. Also, the inner face protection layer is required to have acompounding near to that of the carcass rubber so as not to deterioratethe function inherent to the carcass ply due to the change of thecarcass rubber composition before and after the vulcanization, andfurther a higher adhesion force to the innerliner layer made from thebutyl based rubber composition before and after the vulcanization.

The oxygen trapping function is attained by deteriorating the rubber ofthe inner face protection layer itself through oxidation. However, asthe fracture resistance of the inner face protection layer lowers due tothe deteriorated through oxidation, if a large force is applied to thetire by riding over a projection in the last stage of the tire runningor the like, the inner face protection layer is broken to bring aboutthe tire troubles such as puncture and the like. Therefore, rubberhaving a high fracture resistance even in the deterioration throughoxidation is desirable as a rubber for the inner face protection layer.Also, a rubber having a high elasticity and a high creep resistance isdesirable as the rubber for the inner face protection layer in order toreinforce the carcass ply to suppress the occurrence of the creeping dueto the tire running or the internal tire pressure.

In the pneumatic tire disclosed in Japanese Patent No. 2614441, theinnerliner layer arranged inside the carcass layer has a three-layerstructure of inner layer-middle layer-outer layer in which p-phenylenediamine based antioxidant is compounded in the middle layer at an amountof 5-30 parts by weight based on 100 parts by weight of a rubbercomponent in order to provide a resistance to oxidation deterioration.

In JP-A-2001-121906 is disclosed a technique that an elastic reinforcinglayer is arranged between the carcass ply and the innerliner layer and acopolymer consisting of one or more monomers containing a conjugateddiene and one or more aromatic vinyl monomers is used in thisreinforcing layer.

In JP-A-9-272308 and JP-A-10-297209 is disclosed a technique that theinner face protection layer is rendered into a two-layer structure and acobalt compound of an organic acid is substantially removed from arubber composition constituting a layer located side the innerlinerlayer to improve the resistance to oxidation deterioration of the innerface protection layer.

DISCLOSURE OF THE INVENTION

However, Japanese Patent No.2614441 has a problem that p-phenylenediamine based antioxidant compounded in the middle layer is migratedduring the vulcanization to adversely affect the adhesion between theply cord and the carcass rubber.

In JP-A-2001-121906, there is a problem that the compounding cost of theelastic reinforcing layer becomes higher because of using the copolymerconsisting of one or more monomers containing a conjugated diene and oneor more aromatic vinyl monomers.

Even if the cobalt compound of the organic acid is substantially removedfrom the layer located side the innerliner layer as disclosed inJP-A-9-272308 and JP-A-10-297209, the effect of improving the resistanceto oxidation deterioration in the inner face protection layer is notsufficient, and particularly there is remained a problem that the innerface protection layer is feared to be broken in high-temperature andhumid areas.

It is, therefore, an object of the invention to provide a heavy dutypneumatic tire in which the inner face protection layer arranged betweenthe carcass layer and the innerliner layer controls the creeping of thecarcass rubber and also the resistance to oxidation deterioration issufficiently high to prevent the occurrence of the breakage of the innerface protection layer even when being used in the high-temperature andhumid areas.

The inventor has made various studies in order to achieve the aboveobject and found that the inner face protection layer arranged betweenthe innerliner layer and the carcass layer in the heavy duty pneumatictire is rendered into a two-layer structure of a rubber layer adjacentto the innerliner layer and a rubber layer adjacent to the carcasslayer, and the compounding of the rubber composition is changed betweenboth the rubber layers to separate the functions of both the rubberlayers so that the rubber layer adjacent to the carcass layer has a highcreep resistance and a compounding near to that of the carcass layer soas to suppress the influence due to the difference of the compoundingcomposition to the carcass layer and the rubber layer adjacent to theinnerliner layer has a high fracture resistance even after thedeterioration through oxidation, whereby the creeping of the carcassrubber can be suppressed and the resistance to oxidation deteriorationcan be further improved while maintaining the adhesion property betweenthe ply cored and the carcass rubber, and as a result, the invention hasbeen accomplished.

Namely, the invention lies in;

<1> A heavy duty pneumatic tire comprising a carcass layer, aninnerliner layer and an inner face protection layer arrangedtherebetween, characterized in that the inner face protection layer iscomprised of a rubber layer A adjacent to the innerliner layer and arubber layer B adjacent to the carcass layer, and each of rubber layersconstituting the innerliner layer, the carcass layer, the rubber layer Aand the rubber layer B is compounded with a rubber component, sulfur anda cobalt compound of an organic acid, and an amount of sulfur compoundedsatisfies the following equations (I) and (II):S_(A)<S_(B)≦S_(C)  (I)2≦S_(A)≦4  (II)(wherein S_(A), S_(B) and S_(C) are an amount of sulfur compounded in arubber composition constituting the rubber layer A, rubber layer B andthe carcass layer, respectively, based on 100 parts by mass of therubber component).

<2> A heavy duty pneumatic tire according to the above item <1>, whereinS_(B) is not less than 4.

<3> A heavy duty pneumatic tire according to the above item <1>, whereinan elongation at break of the rubber composition constituting the rubberlayer A is 1.00-1.45 times an elongation at break of the rubbercomposition constituting the rubber layer B.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention will be described in detail below.

The heavy duty pneumatic tire according to the invention comprises apair of bead portions, a pair of side portions, a tread portion, acarcass ply toroidally extending between the bead portions, aninnerliner layer, and an inner face protection layer arranged between acarcass layer constituting the carcass ply and the innerliner layer, inwhich the inner face protection layer is comprised of a rubber layer Aadjacent to the innerliner layer and a rubber layer B adjacent to thecarcass layer. Also, each of rubber compositions constituting theinnerliner layer, carcass layer, rubber layer A and rubber layer B,respectively, is compounded with a rubber component, sulfur and a cobaltcompound of an organic acid.

The rubber layer B plays a role that the rubber layer A and the carcasslayer are separated from each other at a certain distance so that a badinfluence due to the compounding difference between the rubbercomposition constituting the rubber layer A and the rubber compositionconstituting the carcass layer does not exert upon the carcass layer,and a role of reinforcing the carcass ply to suppress the creeping ofthe carcass rubber so as not to stand out ply cords toward an inner faceof the tire.

On the other hand, the rubber layer A is sufficient to have the creepresistance lower than that of the rubber layer B because the creeping ofthe carcass rubber is suppressed by the rubber layer B, so that thecompounding of the rubber composition constituting the rubber layer Agives weight to the fracture resistance after the deterioration throughoxidation and hence the amounts of sulfur and cobalt compound of organicacid compounded can be made lower than those of the rubber layer B, andalso the amount of a vulcanization accelerator compounded can bereduced. In case of giving weight to the fracture resistance, anelongation at break of the rubber composition constituting the rubberlayer A is 1.00-1.45 times, preferably 1.02-1.45 times an elongation atbreak of the rubber composition constituting the rubber layer B.Moreover, oxygen passing through the innerliner layer is trapped withthe rubber layer A, so that it hardly arrives at the carcass ply throughthe rubber layer B.

The rubber layer B according to the invention is made of a rubbercomposition equal or similar to the rubber composition constituting thecarcass layer. In such a rubber composition, natural rubber or a rubberblend containing not less than 70% by mass of natural rubber ispreferable as a rubber component. As the other rubber in case of therubber blend are mentioned a synthetic poly-isoprene rubber, apolybutadiene rubber, a styrene-butadiene copolymer rubber and so on.Also, the cobalt compound of the organic acid to be compounded in theabove rubber composition is usually compounded in the carcass rubber forthe carcass ply reinforcing the ply cord (e.g. steel cord) as anadhesion promoter, and includes, for example, cobalt naphthenate, cobaltstearate, cobalt oleate, cobalt tallolate, cobalt resinate and the like.The compounding amount of the cobalt compound of the organic acid in therubber composition is usually 0.1-2.0 parts by mass based on 100 partsby mass of the rubber component. Furthermore, the rubber compositioncontains 4-7 parts by mass of sulfur based on 100 parts by mass of therubber component as a vulcanizing agent.

In the rubber composition constituting the rubber layer B and thecarcass layer are properly compounded carbon black, a vulcanizationaccelerator, an antioxidant, a softening agent and the like in additionto the aforementioned rubber component, sulfur and the cobalt compoundof the organic acid. As the carbon black are used FEF, HAF or ISAFgrade, and a compounding amount thereof is usually 40-65 parts by massbased on 100 parts by mass of the rubber component.

The rubber composition constituting the rubber layer A according to theinvention has substantially the same composition as in the rubber layerB, but the compounding amount of sulfur is decreased so as to satisfy arelationship for the compounding amount of sulfur as mentioned later inorder to attain the separated functions of the rubber layer A and therubber layer B and improve the fracture resistance after thedeterioration through oxidation.

That is, the compounding amount of sulfur in the rubber compositionconstituting the rubber layer A is necessary to satisfy the followingequations (I) and (II) to the compounding amount of sulfur in eachrubber composition constituting the carcass layer and the rubber layer Bfrom a viewpoint that the fracture resistance is improved after thedeterioration through oxidation.S_(A)<S_(B)≦S_(C)  (I)2≦S_(A)≦4  (II)

In the above formulae, S_(A), S_(B), S_(C) are an amount of sulfurcompounded in a rubber composition constituting the rubber layer A,rubber layer B and the carcass layer, respectively, based on 100 partsby mass of the rubber component.

The inner face protection layer according to the invention is comprisedof the rubber layer A and the rubber layer B having the aforementionedrubber composition to separate the functions of these layers from eachother and sufficiently improve the resistance to oxidation deteriorationin the rubber layer A, whereby there can be provided a heavy dutypneumatic tire considerably improving the durability in the use at warmand humid areas.

In order that the inner face protection layer according to the inventionsufficiently develops the above effect, it is preferable that the rubberlayer A has a thickness of 0.2-2 mm, and the thickness of the rubberlayer B has a thickness of 0.2-2 mm. Also, it is preferable that a totalthickness of the rubber layer A and the rubber layer B is within a rangeof 0.5-3.0 mm.

The following examples are given in illustration of the invention andare not intended as limitations thereof.

EXAMPLES

Tires to be tested in Examples 1 and 2 and Comparative Examples 1 and 2are tires for truck and bus having a tire size of 11R22.5 and a ribpattern, in which rubber layer A and rubber layer B and carcass layerare prepared according to a compounding recipe shown in Table 1.Moreover, the rubber layer A has a thickness of 0.3 mm and the rubberlayer B has a thickness of 0.3 mm.

After each test tire is run on a drum over 100,000 km, the tire is cutto measure elongation at break of rubber in the rubber layers A and B, asinking amount of ply cord and adhesion property to the ply cord by thefollowing measuring methods to thereby obtain results shown in Tables 1and 2.

(1) Elongation at Break of Rubber (According to JIS K6251-1993)

A JIS #3 sample is taken out from portions of the rubber layer A and B,respectively, and then an elongation at break thereof is measured bymeans of an Instron tension testing machine.

(2) Sinking Amount of Ply Cord

After only the innerliner layer is taken out from the tire after therunning, an irregular state of the inner face protection layer isevaluated by an average value of a sinking amount (difference invertical length between mountain portion and valley portion in theirregularity).

(3) Adhesion Property to Ply Cord

An adhesion force between ply cord and carcass rubber is evaluated by anadhesion ratio of the carcass rubber to the ply cord after an innermostply is taken out from a shoulder portion of the tire.

The evaluation results of the tests of the above (1) to (3) are shown inTables 1 and 2 by an index on the basis that Comparative Example 1 is100. Moreover, the larger the index value in any test results, thebetter the property. TABLE 1 Carcass Rubber layer layer B Rubber layer ANatural rubber 100 100 100 100 100 100 N326 (carbon black) 50 50 50 5050 50 Cobalt compound of 1.5 1.5 0 0 0 0 organic acid Zinc white 8 6 6 66 6 Antioxidant 1 1.5 1.5 1.5 1.5 1.5 Vulcanization accel- 0.8 0.8 0.60.6 0.6 0.6 erator Sulfur 5 5 4.5 3.5 2.5 1.5 Elongation at (%) — 400392 440 500 600 break of rubber (index) 100 98 110 125 150

TABLE 2 Comparative Comparative Example 1 Example 1 Example 2 Example 2Sinking amount 100 98 97 90 of ply cord Adhesion property 100 100 100 90to ply cord

In the tires of Examples 1 and 2, the sinking amount of ply cord isequal to that of Comparative Example 1, and also the adhesion propertyof carcass rubber to ply cord is equal to that of Comparative Example 1,while the elongation at break of rubber is largely improved and thefracture resistance is improved. On the other hand, in the tire ofComparative Example 2, the elongation at break of rubber is considerablyimproved to improve the fracture resistance, but since a modulus ofelasticity is too low, the sinking amount of ply cord is deteriorated toa considerable extent, and further the adhesion property of carcassrubber to ply cord lowers.

INDUSTRIAL APPLICABILITY

According to the invention, the durability of the heavy duty pneumatictire can be considerably improved by improving the elongation at breakof rubber in the rubber layer A adjacent to the innerliner layerparticipating to the resistance to oxidation deterioration in the innerface protection layer arranged between the innerliner layer and thecarcass layer in the heavy duty pneumatic tire without deteriorating theadhesion property between ply cord and carcass rubber and the sinkingamount of ply cord.

1. A heavy duty pneumatic tire comprising a carcass layer, an innerlinerlayer and an inner face protection layer arranged therebetween,characterized in that the inner face protection layer is comprised of arubber layer A adjacent to the innerliner layer and a rubber layer Badjacent to the carcass layer, and each of rubber layers constitutingthe innerliner layer, the carcass layer, the rubber layer A and therubber layer B is compounded with a rubber component, sulfur and acobalt compound of an organic acid, and an amount of sulfur compoundedsatisfies the following equations (I) and (II):S_(A)<S_(B)≦S_(C)  (I)2≦S_(A)≦4  (II) (wherein S_(A), S_(B) and S_(C) are an amount of sulfurcompounded in a rubber composition constituting the rubber layer A,rubber layer B and the carcass layer, respectively, based on 100 partsby mass of the rubber component).
 2. A heavy duty pneumatic tireaccording to claim 1, wherein S_(B) is not less than
 4. 3. A heavy dutypneumatic tire according to claim 1, wherein an elongation at break ofthe rubber composition constituting the rubber layer A is 1.00-1.45times an elongation at break of the rubber composition constituting therubber layer B.